Application of the finite element method to the stability analysis of shallow tunnels

A contribution of the finite element method is discussed to the stability analysis of shallow underground excavations, driven in geotechnical media that may be subjected to the phenomenon of strain localisation and to the consequent mechanical strain softening. A purposely designed finite element technique is presented and its application to the simulation of laboratory tests on tunnel small scale models is discussed. The laboratory tests, carried out on 2D plane strain and 3D models, and the adopted materials are first described. Then, the observed phenomenon of strain localisation is singled out and a numerical procedure is suggested based on a possible interpretation of the phenomenon. The numerical simulation of the laboratory tests allows to draw some conclusions, from one side about the effectiveness of the method and its possible refinement and, from the other side, about the influence of the strain localisation on the development of the collapse mechanism and on the evaluation of the minimum confining pressure that stabilizes the tunnel cross section and the tunnel face, for respectively the 2D and the 3D problem.